I hope this is my issue as well. I had I randomly shut down Saturday night 2.5 hours into a carved I grounded the table (it is an old drafting table so it has metal frame which the controller is screwed too.)
I also grounded all the dust collection back to the unit.
In the last picture you can see I have a grounding point at the stepper, when I connected this ground, the motor started making a humming clicking noise. I disconnected it before operation but it was concerning as I wanted to ground the rails. Hopefully someone more knowledgeable than I could explain this.
Also I should I note that my controller was already fed via a UPS on a separate protected circuit.